Digital communication device

ABSTRACT

A digital communication device such as a hearing aid or a headset has a power supply, a signal processing device, means for receiving a wireless signal and a receiver or loudspeaker which produces an audio signal based on a modulated pulsed signal with high frequency shifting rate produced by the signal processing device. The receiver also has a first and a second connection point for receiving the pulsed modulated signal wherein the sound-producing parts of the receiver are at least partially enclosed by a metal box, and a third connection point is provided which is in electrical contact with the metal of the metal box. The third connection point is connected to the electric circuitry of the communication device at a point having a stable and well defined electrical potential. In this way the electro- and magnetic radiation from the metal parts of the receiver is reduced.

AREA OF THE INVENTION

The invention concerns a digital communication device like a hearing aidor a headset. In such devices a receiver or loudspeaker unit is used,which produces an audio signal which is presented to the ear of theuser. In modem digital devices the signal served at the receiver is amodulated pulsed signal with high frequency shifting rate. When a signalof the above kind is served at a loudspeaker, the loudspeaker coil andother electronic parts will function as a filter, such that the audiosignal produced will have a reasonable quality

BACKGROUND OF THE INVENTION

The high frequency shifting rate will also cause a considerableelectromagnetic radiation noise to be produced from the feed lines andfrom the coil in the loudspeaker. This radiation may be picked up byother parts of the communication device, and especially if thecommunication device has a coil or antenna for wireless reception ofsignals this is a problem. This will cause the Noise to Signal Ratio(NSR) from the wireless reception to degrade. The radiation from thewires or feed lines can be reduced by use of coaxial cables. Theinvention is targeted at reducing the radiation from the speaker case.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a digital communicationdevice with improved NSR from the wireless reception of signals. It is afurther object of the invention to provide a hearing aid with a receiverwhich is served with a modulated pulsed signal and a wireless receptionantenna where the NSR of the wireless reception is not seriouslydegraded by electro- and/or magnetic radiation from the signal served atthe receiver.

The proposed solution according to claim 1 attenuates the radiatedfields present in a hearing aid or other communication device.

The solution targets the fields radiated from the speaker. The speakercase in conventional hearing aid or other communication devices is notconnected to any electrical potential in the system and therefore has afloating potential. The speaker case is energised and radiates aconsiderable electrical field in its proximity. Connecting the speakercase to a well defined stable potential almost eliminates the radiatedfield.

Preferably the speaker case is connected to the negative pole of thepower supply.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic representation of a hearing aid with the receiver.

FIG. 2 is a diagram showing the frequency spectrum of the noise signalradiated from the receiver casing.

DESCRIPTION OF A PREFERRED EMBODIMENT

A schematic view of a hearing aid according to an embodiment of theinvention is shown in FIG. 1. The hearing aid comprises a microphone 10and a wireless receiving antenna 8, both connected to a DSP unit 9. TheDSP unit comprises a sigma-delta modulator 7, and also the DSP unit isconnected to a battery 6 and a receiver 1. The receiver 1 is driven by asigma delta modulated signal, which is produced by the modulator 7, andwhen the receiver 1 demodulates this signal, it produces an audiosignal. Two wires 2,3 are required for the actual sigma delta modulatedsignal. A third wire 4 is connected to a metal casing 5 of the receiver1. This wire 4 is grounded, which means that it is connected to theground or the zero potential. The ground in this case is zero potential.Usually, it is defined as the negative battery potential but it couldalso be any other stable voltage potential within the hearing aid.

The radiated noise from the receiver casing and the two wires 2,3 is themodulation noise of the sigma delta modulator 7. The general measuredspectrum is as depicted in FIG. 2. The sigma delta modulator 7 is usedin hearing aids because of its low noise in the audio band compared withordinary quantisation of the audio signal. The disadvantage is thehigher noise in the high frequency range and as can be seen in FIG. 2this noise remains at a high level even at very high frequencies. Thisis generally not a problem for hearing aids where the audio band is ofprime concern. However when the hearing aid is equipped with means 8 forreceiving wireless signals in the same frequency range as the noisesignal, this noise is very disturbing, and it is necessary to applyspecial measures to dampen this noise. By connecting the receiver metalcasing 5 with the zero potential in the hearing aid the noise which isradiated is reduced considerably because the grounded case will act as ashield towards the electrical fields generated inside the case.Furthermore, any magnetic fields inside the case will induce currentsthat will run in the conducting case of the speaker. If the speaker casehas a floating potential large voltages will be generated on the casesurface. This is prevented when the casing is effectively shorted toground by connecting the receiver case to the zero potential.

The connections 2,3 between the receiver and the other parts of thehearing aid, and also the connection 4 between the case and groundpotential could be made by conducting wire. Generally, all 3 wiresshould be as short as possible as these wires also generate noise.Therefore, the resulting resistance should be in the order of a fewmilliohms, preferably below 5 mOhms.

1. Digital communication device like a hearing aid or a headset with apower supply, a signal processing device, means for receiving a wirelesssignal and a receiver or loudspeaker, which produces an audio signalbased on a modulated pulsed signal with high frequency shifting rateproduced by the signal processing device, where the receiver has a firstand a second connection point for receiving the pulsed modulated signalwherein the sound producing parts of the receiver are at least partiallyenclosed by a metal box, whereby a third connection point is providedwhich is in electrical contact with the metal of the metal box andwhereby this third connection point is connected to the electriccircuitry of the communication device at a point having a stable andwell defined electrical potential.
 2. Digital communication device asclaimed in claim 1, wherein the third connection point is connected toground potential, which is the negative pole of the power supply.